How to clip Background Sky Texture to above Horizon
You can use Generated texture coordinate with World to read Z axis on -1 to 1 range. Here -1 is straight under, 1 is straight up and 0 is the horizon. Taking it from there, you could simply use Math node to test for Greater than 0. Math node returns 0 for under 0 and 1 immeadiately above 0 so you can achieve a sharp mask.



For smoother options you could use Color Ramp node, it even conveniently ignores anything below 0 in the range.

EDIT: Here are couple setups for Color Ramp. First ColorRamp range from horizon to Zenith, horizon is on the left of the ColorRamp at value 0.0.



And here we have shifted the -1 to 1 range between 0 and 1 by adding 1 and multiplying by 0.5. Horizon is in the middle of the ColorRamp at value 0.5.